The philosophy of existentialism is undergoing an ecological renewal, as global warming, mass extinction, and other signs of the planetary scale of human actions are making it glaringly apparent that existence is always ecological coexistence. One of the most urgent problems in the current ecological emergency is that humans cannot bear to face the emergency. Its earth-shattering implications are ignored in favor of more solutions, fixes, and sustainability transitions. Solutions cannot solve much when they cannot face what it means to be human amidst unprecedented uncertainty and intimate interconnectedness. Attention to such uncertainty and interconnectedness is what "ecological existentialism" (Deborah Bird Rose) or "coexistentialism" (Timothy Morton) is all about. This book follows Rose, Morton, and many others (e.g., Jean-Luc Nancy, Peter Sloterdijk, and Luce Irigaray) who are currently taking up the styles of thinking conveyed in existentialism, renewing existentialist affirmations of experience, paradox, uncertainty, and ambiguity, and extending existentialism beyond humans to include attention to the uniqueness and strangeness of all beings--all humans and nonhumans woven into ecological coexistence. Along the way, coexistentialism finds productive alliances and tensions amidst many areas of inquiry, including ecocriticism, ecological humanities, object-oriented ontology, feminism, phenomenology, deconstruction, new materialism, and more. This is a book for anyone who seeks to refute cynicism and loneliness and affirm coexistence.